Any resin composition for coating and protecting optical semiconductor elements such as light-emitting diodes (LED) is required to give a transparent cured product. The composition comprising an epoxy resin such as bisphenol-A type or an alicyclic epoxy resin and an acid anhydride curing agent has been generally used. (See Japanese Patent No. 3241338 and JP-A H7-25987.)
The conventional transparent epoxy resin, however, is poor in light resistance on account of its low transmittance for short-wave light or liable to discoloration due to light-induced degradation. To solve this problem, there have been proposed a resin composition for coating and protecting for an optical semiconductor element comprising an alkenyl group-containing silicon compound having at least two carbon-carbon double bonds in one molecule, a silicon compound having at least two SiH groups in one molecule, and a hydrosilylating catalyst. (See JP-A 2001-002922 and WO 2006/77667.)
However, such a cured product of the silicone composition, especially a cured product of the silicone composition having a refractive index of up to 1.45, has a drawback that the cured product has a large gas permeability and hence permit permeation of sulfide gas existing in the storage and operation environment, when compared with the conventional epoxy resin compositions. Thus, the sulfide gas permeating the silicone cured product would react with the silver-plated surface of a lead frame substrate of LED package, thereby changing the plated silver to silver sulfide by sulfide reaction, resulting in blackening of the silver-plated surface.
Recently, LEDs have become brighter than before in proportion to the increasing current supplied to them. This leads to a considerable heat generation from the chip to such an extent that heat dissipation only from the lead frame is not enough to keep the junction temperature low. The resulting high temperature brings about degradation of encapsulated resin and die-bonding resin, which is responsible for short-lived LEDs.
There has recently been proposed a way of solving the above-mentioned problem with sulfidization of silver-plated surface by using a silicone cured product having a refractive index of at least 1.45 which has a comparatively small gas permeability. However, this silicone cured product is still subject to discoloration because the recent light-emitting elements are intended for high emitting efficiency accompanied by high heat generation.
The light-emitting semiconductor device such as white LED is usually composed of a die pad of reflector having a lead frame and a light-emitting element supported thereon, which are encapsulated with a thermosetting resin such as silicone resin containing a fluorescent substance. The light-emitting element such as LED generates intense light and high heat. The light and heat degrade the encapsulated resin near the element although heat dissipates through the lead frame. This phenomenon is conspicuous particularly on the element's surface where there exists the fluorescent substance. The encapsulated resin in contact with the element's surface easily deteriorates because it receives an intense light and accumulates heat.